1. Field of the Invention
The present invention relates to an antenna to be mounted on a travelling equipment such as vehicle or the like.
2. Description of the Prior Art
The recent developments of tele-communication technology have significantly promoted the progressive applications in the fields of wire communication and especially radio communication.
In mobile radio communication systems which are able to perform the simultaneous transmission and reception of information signals such as a duplex communication system in land mobile radio-telephones and the like, the signals are modulated by the use of carriers having different frequencies. The frequencies of the respective carriers for sending and returning signals must be separated from each other sufficiently to prevent interference between two carriers.
Consequently, an antenna mounted on a vehicle must have a sufficiently widened resonant frequency band to contain said two different frequencies for transmission and reception, and it must be small and have a low profile.
In the prior art, the small antenna on the vehicle is frequently in the form of inverted F antenna shown in FIG. 11, which comprises an L-shaped radiator plate 5 having one leg connected electrically and mechanically with a ground plate 2. To make the proper impedance matching, the antenna is fed at a point which is slightly spaced away from the bent portion of the L-shaped radiator (offset feed).
Such an antenna has only a very narrow band width which is of the order of a few percent of the carrier frequency. Due to any external factors, the resonant frequency of the antenna would be frequently shifted to be forced out of the frequency bands covering the transmission and reception frequencies, resulting in interrupted transmission or reception.
Some attempts have been made to overcome the above disadvantage in the prior art. One of these attempts is that an antenna includes an auxiliary plate (sub-radiator) 11 positioned in parallel to the radiator 5, as shown in FIG. 12. The sub-radiator plate 11 is non-feed driven. Another attempt is that an additional plate 7 is positioned adjacent to the radiator 5, as shown in FIG. 13. These improvements are intended to overlap the resonant frequency bands produced by both the radiator and the sub-radiator to provide a more widened resonant frequency band such that it will not be forced away from the transmission and reception frequency band due to any external factor.
A further attempt has been made that an antenna includes an impedance compensating element 12 additionally connected with the feed line of the antenna to increase the resonant frequency band width such that the impedance matching with the feed line is provided (FIG. 14).
However, within the resonant frequency band in the prior antenna, a range in which a return loss is less than a predetermined voltage standing wave ratio (VSWR&lt;2.0), that is, a range in which the return loss is less than -10 db could not be expected over 7%-9% in the fractional band (FIG. 15). Thus, it was not believed that the prior art can provide any improved antenna system having an increased degree of freedom with which the resonant frequency band is sufficiently widened.
In particular, the two way simultaneous transmission and reception system for automobiles, which is intended by the present invention, has an antenna surrounded by various automobile components by which the transmission and reception of the antenna would be adversely affected. It is therefore desired to provide a radio communication antenna which has a widened band width with a degree of freedom for stabilizing the transmission and reception even under the above circumstances.